[infra-sk] Add (empty) type declarations for html-webpack-inject-attributes-plugin.
[infra-sk] Add (empty) type declarations for html-webpack-inject-attributes-plugin.

This is necessary to prevent TypeScript errors due to implicit "any" types in follow-up CLs.

Bug: skia:10292
Change-Id: I2ce8c93b01906b3268002367598fc771ba03e5b5
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/292480
Reviewed-by: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>
Commit-Queue: Leandro Lovisolo <f558143c2568be8d55575d13f30a1fc5cb8009d5@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot